1. Field of the Invention
The present invention relates generally to orthopaedic and orthotic hinge mechanisms which are used in braces of various kinds.
2. Introduction
Orthopaedic and orthotic hinges vary considerably in design and function. They are employed to cross joints, such as the knee or elbow and their function is usually to supplement or partially substitute for, the weight-bearing and motional characteristics of these joints. They are generally used in pairs with one hinge fitted laterally and the other fitted medially across the joint.
Orthopaedic and orthotic hinges are of two main types. The first employs a single pivot and is generally described as uniaxial or unipivotal; this type is quite commonly used in knee braces fitted in the practice of sports medicine often following damage to the ligaments of the knee.
Uniaxial hinges are very commonly fitted to heel cups which is the common name applied to the terminal element in a cast brace. More recently, uniaxial hinges have been used in lower leg walking devices for fractures of the foot: these are used instead of short leg walking casts.
The biomechanics of ankle motion are complex but the therapeutic requirements is normally to provide only plantarflexion/dorsiflexion; this is why uniaxial hinges can be used. In conditions such as ruptured Achilles tendon which has undergone surgical repair it is important to hold the foot in marked plantarflexion for part of the treatment period. This could be achieved with a device having a lockable uniaxial hinge.
Uniaxial hinges have also been used at the hip but when used alone they can only track the flexion/extension axis. Since the hip is a ball and socket joint capable of compound adductive/abductive motion and flexion/extension motion i.e. circumduction, such hinges are clearly of limited value.
In biomechanical terms, single pivot hinges are unphysiological when used at the knee because the knee does not move as a simple pivot.
The second type of hinge mechanism is very widely used in cast bracing hinges and knee braces. In this type there is a central mount or plate which bears two pivots. The hinge arms are mounted, one on each pivot and each arm has gear teeth at its extreme end such that when mounted upon its pivot, the teeth mesh with that of its neighbour. The effect of this is that when one hinge arm is moved, the other hinge arm must move also.
In mechanical terms a geared two pivot mechanism loses one degree of freedom. Geared two pivot hinges are also unphysiological when used at the knee because they effectively offer a single pivot point which migrates backwards when the hinge is moved from the fully extended to the fully flexed condition. In commercially available types, this migration is about 8 mm.
Knee motion involves forward sliding of the femoral condyles over the tibial plateau as the knee extends from the fully flexed position. As the knee moves into the last 20 degrees or so of extension, a marked pivotal element is introduced and over the last 5 or so degrees of extension there is a rotational `screwhome` component.
A third type of orthopaedic hinge does offer good tracking of the human knee joint by employing two truly independent pivots in conjunction with a stop incorporated in the hinge body to arrest uncontrolled anterior motion of the femoral condyles over the tibial plateau in a cruciate ligament deficient knee. This type of hinge is called a true bi-pivotal hinge or by some authors a true bi-axial hinge.
3. The Prior Art
Although several manufacturers employ single pivot hinges in heel cups these do not usually have adjusting or locking means.
Single pivot knee hinges are now used much less since uniaxial hinges cannot track the human knee satisfactorily. If the brace in which the hinge is incorporated fits snugly, the hinge will impart undesirable forces to the joint. Consequently, although the motion control mechanism Housewerth describes for a `conventional` leg brace in U.S. Pat. No. 4,620,586 is clever and compact, the unfaxial hinge he uses is not suitable for physiological knee bracing applications.
A single pivot hinge is used in a knee brace supplied by Messrs Medical Designs Inc of Arlington, Tex., U.S.A. This design is disclosed in U.S. Pat. No. 4,489,718 to Martin and employs a slidable pivot and a stop peg limited by flexible plungers.
The hinge axis appears to be able to slide only about 7 mm, conferring only a minimal advantage is minimal since the instant centre pathway of the knee, during the sliding pase of flexion, can be shown to migrate considerably more than this and not in a straight line. The instant centre pathway of the knee is the locus of the effective knee axis as the knee joint moves through a complete cycle of flexion and extension.
The adjustment mechanism allows locking and limited ranges of motion. Long springs, carried within slots in one element of the hinge body, are driven by grub screws. A stop rivet which engages another element of the hinge body, extends into the slots occupied by the springs. Depending upon whether the springs are both driven up against the rivet or are parked some distance away from it, the hinge may be locked or allowed limited motion.
Although this adjustment mechanism is effective, the hinge body necessary to contain it confers a considerable weight and size penalty. The hinge body element containing the adjusters is about 4.25" long and about 1.75" wide, compared with 3.25" by 2.25" for another adjustable hinge body described below (Lerman) and 2.25" by 1.25" for the Protectair hinge also described below.
Commercially available geared two-pivot hinges have teeth around the edge of the pivot end of the hinge arm. Although each arm is pivoted separately in the hinge body, independent motion is prevented by mutual engagement of the gear teeth on each arm. The effect of this gearing mechanism is thus to reduce the motional capability of the hinge to a uniaxial device which flexes and extends about the point of engagement of the gear teeth, the hinge body moving posteriorly and anteriorly respectively, relative to said point of engagement.
Various embodiments of this basic hinge design allow controlled mobility and locking. For instance, in some types, a drop lock is used to fix the hinge in the fully extended position. This comprises a metal ring-piece fitted around one hinge arm. The ring can either be parked, away from the body of the hinge mechanism or it can be slid down the arm and over part of the hinge body is such a way that the arm is trapped. The ring can be locked in this latter position by means of a grub screw. By trapping the hinge arm against the body, all motion is prevented and the hinge becomes a straight strut.
In another variant, an arcuate slot in one part of the hinge body, lies over a tapped hole in one hinge arm. A threaded peg screws into the tapped hole in the hinge arm and acts as a stop. This arrangement gives the hinge a fixed limited range of motion determined by the angular dimension of the arcuate slot.
In U.S. Pat. No. 4,337,764 and published European Application No. 821015955 Lerman discloses an adjustment mechanism for two-pivot geared hinges. The system depends on a hinge backplate with an arcuate slot in which are located two compression screw sets lying outside either side of the hinge arm. In commercially available versions of this device, such as those hinges supplied in the USA by Messrs United States manufacturing Company of Pasadena, Calif., there are two such slots and a total of four compression screw sets.
Tests which we have carried out using an Instron machine, show that this mechanism is liable to slip at physiological loads and that this slippage occurs in an unpredictable manner.
In U.S. Pat. No. 4,599,998 to Castillo, a motion control system based on a ratchet mechanism is disclosed for geared two pivot hinges which is neat and compact but limited to that type of hinge only.
A different type of adjustment mechanism for a two-pivot geared hinge is used in a design supplied by Messrs Rolyan Manufacturing Co Inc of Menomonee Falls, Wis., U.S.A. In this device, two screws located in the top of the hinge body are used to limit travel of one hinge arm in flexion and extension respectively.
This is achieved by driving the screws down into the body so that the ends strike the top edges of the hinge arms. The screws remain exposed at all times and require a locking nut to maintain adjustment. The hinge may be free or locked in one position or set for limited ranges of motion.
We know of very few commercially available examples of true bipivotal hinges and we have found relatively few references in the art via patent searches.
We know of a sports brace made by Omni Scientific of Martinez, Calif. This is believed to be based on a patents granted to Anderson who has both a U.S. Pat. No. 4,249,524 and a PCT patent WO 82/02658.
Anderson teaches a bi-pivotal hinge in his US patent but we believe that this has unphysiological characteristics. This is because the pivots are very widely spaced and shortening could occur in the hinge as it flexes, leading to effective shortening of the cast or brace in which it is used. This allows the knee joint or limb to `piston` which is undesirable, especially in a damaged knee or in a knee which has recently undergone surgical repair or in a leg where there is a fracture.
The hinge illustrated in Anderson's PCT patent, granted a year later than the US patent, is different insofar as the headplates are concerned but judging from the drawings he still discloses widely separated pivots.
In effect, the hinge centre bar in the US patent fulfils the function of both a mounting for the pivots and of hinge arms since it extends as far as the members normally regarded as headplates. In the PCT patent, so far as we can judge, the hinge centre bar appears to extend quite close to the headplates.
We believe that wide pivotal spacing is of significance in regard to `pistoning` and will in addition detract from proper function of a hinge incorporated in a brace. For instance, where wide pivotal spacing is employed, the medial collateral ligament will receive little, if any, protection from the brace against a lateral blow when the knee is moderately flexed.
Although Anderson briefly mentions stops in his PCT patent he does not disclose a proper motion control system in any detail. In any such system, the interrelationship between the control of motion and the pivot spacing in true bi-pivotal hinges is important.
In 1983 we introduced into the European market, a true bi-pivotal hinge with closely spaced pivots. This is made by Messrs Protectair Limited of Stokenchurch, Buckinghamshire U.K. and is sold under the name Sheffield System. It features a hinge body with a metal chassis having arcuate slots at each end.
Compression screws, fitted with washers and nuts, lie in the slots and the positions of the compression screw sets may be varied over a wide range. Unlike the Lerman hinge, the compression screws do not act against either side of the hinge arms, instead they abut a pin rivetted to each hinge arm, centrally, under the arcuate slots. Like the Lerman hinge adjustment mechanism, this system has also been found on Instron testing, to slip at physiological loads. However, when accessory locking plates supplied for use with this design, were fitted, slippage was greatly reduced.
We have confirmed, by means of combined video, computer and force plate gait analysis, that our bi-pivotal knee hinges with closely spaced pivots introduce less disturbance to the normal gait (or walking pattern) than either geared two pivot hinges or single axis hinges.
Furthermore, we have confirmed at Sheffield and Brunel Universities, that with the spacing of the pivots we have used in bi-pivotal hinges, the instant centre pathway of the knee during the flexion/extension cycle can be almost entirely accommodated in the majority of adults.
Also, pistoning does not occur in such hinges as those immediately hereinbefore described when the knee is under load (provided the hinges are fitted properly).
Attention should be drawn to U.S. Pat. No. 4,520,802 granted to Mercer and Aaserude which teaches another bi-pivotal hinge featuring wide pivot spacing. These authors' principal disclosure is, however, their motion control system based on indexing blocks. The system they describe is discontinuous and leaves the user subject to the values on the index blocks made available by the manufacturer. In addition, the time taken to remove parts of the device and to select and substitute accessories would be considerable and not appropriate to busy clinics and doctors' offices where there is usually the need to process numerous patients efficiently and quickly. As taught, the intention seems primarily to provide flexion control.
Most hinges have securing means for fixing them directly or indirectly to a limb. Nowadays there is an increasing trend towards the use of devices which are retained on the limb by means of several straps. Where this method is employed, the hinge mechanism will usually have arms fitted with curved plates which are often called shells.
Where the hinge mechanism is to be retained on the limb by a cast, it will usually have hinge arms which terminate in structures adapted for embedding in the cast and frequently termed headplates or anchor plates.
Orthotic hinges are normally supplied as independent units which are subsequently either built directly onto plastic orthoses or fitted to mating side arms called `steels` and incorporated into calipers. Lower limb orthoses in particular are generally secured to the limb with straps.
Observations made under widely varying conditions in several different countries lead us to the conclusion that strap-on devices, especially for the lower limb, almost always have more potential for relative motion between the limb and the device than do casts. This is primarily because casts are inherently rigid and constitute a fully circumferential integrated structural unit, whereas strap on devices are usually made from a combination of soft goods and flexible materials and cannot form an integrated circumferential structure.
It is important to understand, therefore, that in the design of motion control mechanisms for orthopaedic and orthotic hinges adjustments should be capable of continuous variation. This ensures that proper compensation for relative motion between the leg and the brace when such hinges are used with strap-on braces can be achieved.
Somewhat paradoxically, it is in the damaged but otherwise normal knee that the greatest attention to accurate tracking is necessary, since, with well managed rehabilitation, very good results can be obtained. However, with grossly damaged knees ravaged either by disease or birth defect and in which there is no hope of normal motion, a less complex hinge may often be used and other factors in the design of the device, such as locking mechanisms may be more important.
In published UK Pat. Application Number 2,163,352 and in U.S. Pat. Application No. 734,050 we disclosed a hip hinge capable of circumduction and of being locked in a number of abducted positions. We know of a single axis hip hinge formerly manufactured by Messrs Blatchfords of Basingstoke, UK and we have seen hinges according to Lerman modified for use as single axis hip hinges, thus providing a locking and limiting facility but on the flexion axis only.